Perhaps the largest visible change in the history of audio is the replacement of Long Playing Records (LPs) to their successor the Compact Disk (CDs). LPs are considered “Analog” recordings where the stored acoustic signals of the recordings are physical disturbances to a smooth spiral pressed into vinyl. CDs store the recordings as digital values that vary in time. The CD is a standard where the encoded values have 16-bit resolution and are recorded at a rate of 44.1 kHz. There were obvious advantages to the CD, but for many of the highest performing audio systems, the LP has yet to be surpassed by the CD in terms of fidelity of the playback.
There are two reasons for the lack of fidelity of CDs: Digital audio recordings have a distortion profile where the distortion is lowest for the loudest signals, whereas analog recordings have exactly the opposite behavior, the distortion increases with signal amplitude. Much of music is relatively low level with occasional loud signals. Hence, digital audio recording sounds best when the music is loud and analog recordings sound best when the music is quiet.
The second reason why Analog Audio can sound better than Digital Audio is due to the “Pink” nature of music. Specifically, as shown in FIG. 14, a spectrum analysis of typical music illustrates that the high frequency spectrum is much lower in amplitude than the low frequencies. In other words, the high frequencies are always very quiet. Typically, the amplitude is inversely proportional to the frequency. 10 KHz content is typically two orders of magnitude smaller than 100 Hz content.
CD audio is based on a 16-bit digitization, so the largest number for a quantization is 32768 (the smallest is −32768). Typically a signal has 20 dB of headroom (20 dB is a factor of 10), so 3277 is a typical quantization for a normal passage of music. If the 100 Hz content were 3277, the 10 kHz would have 2 orders of magnitude less, or 30 quantization levels (out of a maximum of 32768). This is obviously not the highest resolution.
All good recording methods previously in history have included equalization to ensure high frequency fidelity. For example, LPs used Recording Industry Association of America (RIAA) equalization, and Analog Tape used the National Association of Broadcasters (NAB) standard. The high frequencies are boosted (emphasis) prior to storage. A reverse equalization is applied during playback (de-emphasis). As Digital engineers were anxious to rid the system of precision capacitors needed to accurately apply emphasis and de-emphasis, emphasis was essentially excluded from digital recordings. There is an equalization curve defined for CD, it is 10 dB (about a factor of 3). This was used in very early recordings. This was not seen as helpful and was quickly excluded. The RIAA equalization curve had nearly 40 dB of high frequency emphasis wherein 40 dB is a factor of 100.
FIG. 1 illustrates a block diagram of a typical audio processing element 100. Specifically, as shown in FIG. 1, the audio processing element 100 comprises an interpolation component 102, a digital to analog converter 104 and an output filter 106 operably coupled to each other. The interpolation component 102 receives an input audio signal 99 from an audio source (not shown) such as the analog low amplitude 5 KHz sine wave shown in FIG. 7 after it has been converted to a digital signal including the associated quantization error as shown in FIG. 8. Such a low amplitude high frequency sine wave is a typical component in a digital music recording such as a CD recording. The interpolation component 102 then interpolates the input audio signal 99 in order to produce the interpolated audio signal 98. FIG. 9 illustrates the sine wave of FIG. 8 after a standard 8× interpolation. The quantization error is apparent in the difference between the input signal 99 and the interpolated audio signal 98. The digital to analog converter 104 then receives and converts the interpolated audio signal 98 to an analog signal 97, which is then filtered by the output filter 106 in order to produce the output audio signal 96 that is transmitted to an audio playback device (not shown).